In a general TOFMS, ions accelerated by an appropriate electric field are injected into a flight space where no electric field or magnetic field is present. The ions are separated by their mass to charge ratios according to the flight time until they reach and are detected by a detector. Since the difference in the flight time of two ions having different mass to charge ratios is larger as the flight path is longer, it is preferable to design the flight path as long as possible in order to enhance the resolution of the mass to charge ratio of a TOFMS. In many cases, however, it is difficult to incorporate a long straight path in a TOFMS due to the limited overall size, so that various measures have been taken to effectively lengthen the flight length.
In the Japanese Unexamined Patent Publication No. H11-135060, a TOFMS is disclosed in which an “8” shaped loop orbit is formed, and ions are guided to fly the loop orbit many times so that a long flight path is achieved.
A problem of such an orbit construction is explained using FIG. 4, which illustrates a simple circular orbit instead of an “8” shaped loop orbit for simplicity.
An ion starting the ion source 1 is introduced into the flight space 2 by the gate electrodes 4, and guided along the loop orbit A in the flight space 2. For the visibility of FIG. 4, the electrodes for producing the electric fields to guide the ion along the loop obit A is omitted. After flying the loop orbit A either once or many times, the ion leaves the loop orbit A when it passes the gate electrodes 4 to which an appropriate departing voltage is applied. After leaving the loop orbit A, the ion arrives at the detector 5, where the ion is detected, and the arriving time is measured. Since the flight distance of the ion is longer as the number of turns in the loop orbit A is greater, the difference in the flight time of ions having different mass to charge ratios becomes larger, and it becomes easier to discriminate between ions having close mass to charge ratios. But it sometimes happens that ions having smaller mass to charge ratios catch up with ions having larger mass to charge ratios while they turn the loop orbit A a number of times, and both ions enter the detector almost at the same time, since ions having smaller mass to charge ratios move faster.
It means that, in such a TOFMS, ions having smaller difference in the mass to charge ratio can be adequately separated, but ions having larger difference in the mass to charge ratio are sometimes difficult to separate. In order to avoid such a situation, conventionally the voltage applied to the gate electrodes 4 is controlled so that the mass to charge ratios of ions introduced into the loop orbit A are limited within a certain range. This prevents ions having large difference in mass to charge ratio being detected in a measurement. When ions having a wide range of mass to charge ratios, i.e. from smaller mass to charge ratios to larger mass to charge ratios, are to be measured, several measurement should be repeated to cover the range. Unless enough amount of sample is available, it is impossible to measure the whole range of mass to charge ratios.
Instead of using a loop orbit, the flight distance of ions can be made longer by reciprocating ions along a linear or curved path. But the same problem as discussed above occurs in such a case.